DNA-DNA hybridization studies involving reactions between labeled T-even phage DNA fragments and excess unlabeled T-even phage DNAs gave results which indicated that (a) as expected, close genetic similarities among the T2-T4 and T6 DNAs exist; (b) because reciprocal experiments (labeled T2 DNA reacted with T4 DNA and labeled T4 DNA reacted with T2 DNA, etc.) gave unequal values, different molecular weights are predicted for each of the three phage DNAs. Relative to the molecular weight of T2 DNA the molecular weights of T6 and T4 are 8 and 10% larger, respectively; (c) the differences of the thermal stabilities of the hybrid duplexes denote a closer genetic relationship between the T2 and T6 DNAs than to the T2 and T4 or T4 and T6 DNAs.